Interpretive Summary: While we know terrestrial ecosystems can respond directly to both rising levels of atmospheric CO2 as well as to warming, our knowledge of the combined effects of CO2 and warming on ecosystems is limited by a lack of experiments that have considered both factors in a single experiment. This paper evaluates how knowledge gained from single factor experiments manipulating either CO2 or temperature can be used to predict how ecosystems will respond to future changes in both factors. The results suggest that while important knowledge can be gained from single factor experiments, such experiments often overestimate ecosystem responses of critical attributes like plant productivity and nutrient cycling to the combined effects of rising CO2 and warming. Long-term experiments in which more than one global change factor can be manipulated at a time are needed to clarify findings from single-factor experiments, and to help predict how ecosystems will respond to the multitude of environmental factors involved in climate change.

Technical Abstract:
In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([CO2]) and temperature has illustrated the importance of multi-factorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multi-factorial experiments, this paper synthesizes how ecosystem productivity and soil processes respond to combined warming and [CO2] manipulation, and compare with those obtained in single factor [CO2] and temperature manipulation experiments. Across all combined elevated [CO2] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [CO2]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the CO2-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [CO2]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [CO2] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [CO2] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because we found a stronger similarity in response patterns between combined treatments and single factor [CO2] treatments, our results also suggest that projected responses to global warming in Earth System models should not be parameterized using single factor warming experiments.